Scatter the fennel seeds in a small frying pan over medium-high heat and dry-roast until they start to pop. Transfer to a mortar and crush. Place the crushed seeds in a small bowl and add the remaining ingredients. Mix well.
On a plate break the mozzarella roughly and smear with the marinade and set aside for 15 to 30 minutes.
To serve; Spread the arugula salad on a serving dish. Place the mozzarella over it and cut the tomatoes and place them alongside the dish. Drizzle with some extra olive oil and serve